The attempts are foiled due to threat monitoring software installed in October 2013. However, from June to October 2013, her server is not protected by this software, and there is no way of knowing if there are successful attacks during that time.
A 2014 email from an employee of SECNAP, the company that makes the threat monitoring software, describes four attacks. But investigators will later find evidence of a fifth attack from around this time. Three are linked to China, one to South Korea, and one to Germany. It is not known if foreign governments are involved or how sophisticated the attacks are.
Clinton had ended her term as secretary of state in February 2013, but more than 60,000 of her emails remained on her server. (The Associated Press, 10/7/2015)
In March 2013, a Romanian hacker nicknamed Guccifer discovered Clinton’s private email address and the exact address was published in the media.